I'm genuinely curious... are there viruses that spread in OSX? Not trojans or whatnot that require users to click on files they shouldn't, but rather actual viruses that make use of vulnerabilities in OSX to spread from computer to computer, either via Internet or thumb drives or something? In other words, if I always follow responsible practices (never opening files from untrusted sources), has there been any threat…
I've cleaned IRC bots off of people's OS X installations. They had SSH turned on, with username/pw combos like "bob"/"bob", and public IP addresses.
But you really have to intentionally make multiple poor security choices for something like this happen.
There are also quite a few remote code execution attacks against OS X - you can find details of these in Apple's security bulletin list: